Keys and key signatures

A key is the note a piece treats as home, plus the scale built on it. A key signature is the shorthand at the start of every staff that tells you which notes are sharp or flat throughout.

Why the signature exists

The major scale pattern only lands on white keys from C. From G, the seventh note has to be F♯ or the pattern breaks. From D you need F♯ and C♯. Rather than mark those accidentals on every single note, notation states them once, at the left of the staff.

So a key signature is not decoration. It is a compression trick.

The order is fixed

Sharps always appear in the order F C G D A E B. Flats appear in exactly the reverse: B E A D G C F. Not convention for its own sake — it falls straight out of stacking fifths, which is what the circle of fifths draws.

Key signatures covers how to read one and how to name the key from it in a couple of seconds.

Relative keys

Every major key shares its signature with a minor key a minor third below. C major and A minor both have no sharps or flats. They use the same notes and sound completely different, because they treat different notes as home.